This invention relates to a frequency detection apparatus, and, in particular, relates to an apparatus for detecting the frequency of an input signal with the aid of a reference signal of a predetermined frequency.
Detecting the frequency of the input signals to the monitor control circuit of a multi-frequency monitor system is a basic requirement for operating the monitor system.
According to the conventional approach, there are two methods for detecting the frequency of the input signals to the multi-frequency monitor system.
The first method uses a frequency-voltage conversion circuit to convert the input signal to be detected into a direct current (DC) voltage. Then a plurality of comparators are utilized to compare the DC voltage with a plurality of preset reference voltages. Therefore, through the outputs of the plurality of comparators, the range of frequency of the input signal is determined. However, this method can only detect to which frequency range the input signal belongs. It is also time consuming to preset the plurality of reference voltages for the reference inputs of the plurality of comparators.
The second method uses a microprocessor to periodically access an output value of a counter, which is adapted to receive the input signal, and to calculate the frequency of the input signal periodically based on the output value. However, this method uses a substantial amount of processor time bringing about a low efficiency use of the microprocessor. This is specially true when the frequency of the input signal is not changed for a substantial period of time and the periodical calculation of the frequency by the microprocessor is not necessary at all for this period of time.